U.S. patent number 8,279,518 [Application Number 12/690,253] was granted by the patent office on 2012-10-02 for optical amplifier.
This patent grant is currently assigned to Fujitsu Limited. Invention is credited to Futoshi Izumi.
United States Patent |
8,279,518 |
Izumi |
October 2, 2012 |
Optical amplifier
Abstract
An optical amplifier apparatus for amplifying a wavelength
division signal light includes a detector for detecting an inputted
wavelength division signal light, a dispersion compensator for
compensating for a dispersion of the inputted wavelength division
signal light, an optical amplifier for amplifying the inputted
wavelength division signal light after compensation by stimulated
emission of an optical gain medium including a rare-earth element,
a propagation delay detector for detecting a propagation delay time
of the wavelength division signal light between the detector and
the optical amplifier, and a controller for controlling the gain of
the optical amplifier on the basis of the propagation delay time
such that the change of the gain of the optical amplifier is
adjusted by the propagation delay time.
Inventors: |
Izumi; Futoshi (Kawasaki,
JP) |
Assignee: |
Fujitsu Limited (Kawasaki,
JP)
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Family
ID: |
40350473 |
Appl.
No.: |
12/690,253 |
Filed: |
January 20, 2010 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100118387 A1 |
May 13, 2010 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/JP2007/065841 |
Aug 14, 2007 |
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Current U.S.
Class: |
359/337;
359/341.41; 359/337.5 |
Current CPC
Class: |
H04B
10/2942 (20130101); H04B 10/25133 (20130101); H01S
3/2308 (20130101); H01S 3/1305 (20130101) |
Current International
Class: |
H04B
10/17 (20060101); H04B 10/12 (20060101) |
Field of
Search: |
;359/337,341.41,337.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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8-179388 |
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Jul 1996 |
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JP |
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9-200145 |
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Jul 1997 |
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JP |
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11-330595 |
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Nov 1999 |
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JP |
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2002-261364 |
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Sep 2002 |
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JP |
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Other References
English language International Search Report for PCT/JP2007/065841,
mailed Sep. 11, 2007. cited by other.
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Primary Examiner: Bolda; Eric
Attorney, Agent or Firm: Staas & Halsey LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a continuation of International Application No.
PCT/JP2007/065841 filed on Aug. 14, 2007, the entire contents of
which are incorporated herein by reference.
Claims
What is claimed is:
1. An optical amplifier apparatus for amplifying a wavelength
division multiplexed signal light, the optical amplifier
comprising: a detector configured to detect a wavelength division
multiplexed signal light; a dispersion compensator configured to
compensate for a dispersion of the wavelength division multiplexed
signal light after being detected by the detector; an optical
amplifier configured to amplify the wavelength division multiplexed
signal light after the wavelength division multiplexed light is
compensated for dispersion by the dispersion compensator, the
optical amplifier amplifying the wavelength division multiplexed
signal light by providing pump light to an optical gain medium
including a rare-earth element so that the pump light travels
through the optical gain medium as the wavelength division
multiplexed light travels through the optical gain medium; a
propagation delay detector configured to detect a propagation delay
time of the wavelength division multiplexed signal light between
the detector and the optical amplifier, and to subtract a
relaxation time from the detected propagation delay time to thereby
provided an adjusted propagation delay time, the relaxation time
being a period of time in which electrons of the rare-earth element
fall to an energy level causing stimulated emission with the pump
light in the optical gain medium; and a controller configured to
control a gain of the optical amplifier, based on the adjusted
propagation delay time such that a change of the gain of the
optical amplifier is adjusted by the adjusted propagation delay
time.
2. The optical amplifier apparatus according to claim 1, wherein
the optical amplifier detects the wavelength division multiplexed
signal light input to, and output from, the optical amplifier, and
performs automatic gain control based on the detected wavelength
division multiplexed signal light input to, and output from, the
optical amplifier.
3. The optical amplifier apparatus according to claim 2, wherein
the propagation delay detector detects the propagation delay time,
based on the wavelength division multiplexed signal light detected
by the detector and the detected wavelength division multiplexed
signal light input to the optical amplifier.
4. The optical amplifier apparatus according to claim 3, wherein
the pump light travels through the optical gain medium in a same
direction as the wavelength division multiplexed signal light.
5. The optical amplifier apparatus according to claim 2, wherein
the propagation delay detector detects the propagation delay time,
based on the wavelength division multiplexed signal light detected
by the detector and the wavelength division multiplexed signal
light output from optical amplifier.
6. The optical amplifier apparatus according to claim 5, wherein
the pump light travels through the optical gain medium in an
opposite direction than the wavelength division multiplexed signal
light.
7. An optical amplifier apparatus for amplifying a wavelength
division multiplexed signal light, the optical amplifier
comprising: a first optical amplifier configured to amplify the
wavelength division multiplexed signal light; a dispersion
compensator configured to compensate for a dispersion of the
wavelength division multiplexed signal light after being amplified
by the first optical amplifier; a second optical amplifier
configured to amplify the wavelength division multiplexed signal
light after being compensation for dispersion by the dispersion
compensator, the second optical amplifier amplifying the wavelength
division multiplexed signal light by stimulated emission of an
optical gain medium including a rare-earth element; a propagation
delay detector configured to detect a propagation delay time of the
wavelength division multiplexed signal light between the first
optical amplifier and the second optical amplifier, and to subtract
a relaxation time from the detected propagation delay time to
thereby provided an adjusted propagation delay time, the relaxation
time being a period of time in which electrons of the rare-earth
element fall to an energy level causing stimulated emission in the
optical gain medium; and a controller configured to control a gain
of the second optical amplifier, based on the adjusted propagation
delay time such that the first optical amplifier and the second
optical amplifier provide a constant total gain, and to adjust a
change of the gain of the second optical amplifier by the adjusted
propagation delay time.
8. The optical amplifier apparatus according to claim 7, wherein
the first optical amplifier detects the wavelength division
multiplexed signal light input to, and output from, the first
optical amplifier, and performs automatic gain control of the first
optical amplifier based on the detected wavelength division
multiplexed signal light input to, and output from, the first
optical amplifier, and the second optical amplifier detects the
wavelength division multiplexed signal light input to, and output
from, the second optical amplifier, and performs automatic gain
control of the second optical amplifier based on the detected
wavelength division multiplexed signal light input to, and output
from, the second optical amplifier.
9. The optical amplifier apparatus according to claim 7, wherein
the propagation delay detector detects the propagation delay time,
based on the wavelength division multiplexed signal light input to
the first optical amplifier and the wavelength division multiplexed
signal light input to the second optical amplifier.
10. The optical amplifier apparatus according to claim 8, wherein
pump light pumps the optical gain medium of the second optical
amplifier to amplify the wavelength division multiplexed signal
light from a same direction through which the wavelength division
multiplexed signal light travels through the optical gain
medium.
11. The optical amplifier apparatus according to claim 8, wherein
the propagation delay detector detects the propagation delay time,
based on the wavelength division multiplexed signal light input to
the first optical amplifier and the wavelength division multiplexed
signal light output from the second optical amplifier.
12. The optical amplifier apparatus according to claim 8, wherein
pump light for pumping the optical gain medium of the second
optical amplifier pumps the wavelength division multiplexed signal
light from an opposite direction of which the wavelength division
multiplexed signal light travels through the optical gain
medium.
13. The optical amplifier apparatus according to claim 7, wherein
the first optical amplifier includes an optical gain medium
including a rare-earth element, and the first optical amplifier
causes stimulated emission of the first optical amplifier by adding
a pump light to the optical gain medium of the first optical
amplifier.
14. The optical amplifier apparatus according to claim 13, wherein
the controller synchronizes pump light added by the first and the
second optical amplifiers, based on the adjusted propagation delay
time.
15. The optical amplifier apparatus according to claim 7, wherein
the controller adjusts the change of the gain of the second optical
amplifier by the adjusted propagation delay time after a change of
the gain of the first optical amplifier.
Description
FIELD
An aspect of the embodiments discussed herein is directed to an
optical amplifier for amplifying light in an optical
wavelength-division multiplexing system.
BACKGROUND
Pulses of optical signals used in optical wavelength-division
multiplexing systems experience chromatic dispersion under the
effect of optical fibers serving as optical transmission lines.
Accordingly, a configuration is known in which optical relay
stations and optical receiving stations include dispersion
compensators for compensating for chromatic dispersion of optical
pulses. Dispersion compensators differ in the amount of dispersion
compensation required, depending on the lengths of optical
transmission lines up to optical relay stations and optical
receiving stations. Differences in the amount of dispersion
compensation appear as propagation delays in dispersion
compensators. That is, dispersion compensators provided in optical
relay stations and optical receiving stations have different
propagation delays.
Optical relay stations and optical receiving stations in optical
wavelength-division multiplexing systems also include optical
amplifiers for amplifying light attenuated through optical
transmission lines. Optical amplifiers are known to have different
response control timings of optical amplification due to
differences in the wavelengths of pump lasers for pumping optical
gain media in the optical amplifiers. In particular, a 980 nm pump
laser, which is capable of optical amplification with superior
noise characteristics, has a relaxation time in which excited
electrons responsible for pumping fall to an energy level where
they cause stimulated emission. Accordingly, if a variation occurs
in the number of optical wavelengths used in an optical
wavelength-division multiplexing system, gain adjustment in the
optical gain medium lags behind the variation in the optical power
input to the optical amplifier. This causes a problem in that a
decreased number of optical signal wavelengths results in an
excessively high gain per optical signal wavelength.
As a measure against the relaxation time of an optical amplifier,
Japanese Laid-open Patent Publication No. 2002-261364 discusses
that received light is delayed through a delay line before being
input to an optical amplifying unit.
SUMMARY
According to an aspect of an embodiment, an optical amplifier
apparatus for amplifying a wavelength division signal light
includes a detector for detecting an inputted wavelength division
signal light, a dispersion compensator for compensating for a
dispersion of the inputted wavelength division signal light, an
optical amplifier for amplifying the inputted wavelength division
signal light after compensation by stimulated emission of an
optical gain medium including a rare-earth element, a propagation
delay detector for detecting a propagation delay time of the
wavelength division signal light between the detector and the
optical amplifier, and a controller for controlling the gain of the
optical amplifier on the basis of the propagation delay time such
that the change of the gain of the optical amplifier is adjusted by
the propagation delay time.
The object and advantages of the invention will be realized and
attained by means of the elements and combinations particularly
pointed out in the claims.
It is to be understood that both the foregoing general description
and the following detailed description are exemplary and
explanatory and are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating an optical amplifier applied to an
optical communication system;
FIGS. 2A-2D are time charts of an embodiment;
FIG. 3 is a diagram illustrating a first configuration of the
optical amplifier;
FIG. 4 is a diagram illustrating the configuration of a
variation-holding circuit;
FIG. 5 is a diagram illustrating the configuration of a
microprocessor and a laser controller;
FIG. 6 is a diagram illustrating a second configuration of the
optical amplifier;
FIG. 7 is a diagram illustrating a third configuration of the
optical amplifier;
FIG. 8 is a diagram illustrating a fourth configuration of the
optical amplifier;
FIG. 9 is a diagram illustrating a fifth configuration of the
optical amplifier;
FIG. 10 is a diagram illustrating a sixth configuration of the
optical amplifier;
FIG. 11 is a diagram illustrating a seventh configuration of the
optical amplifier; and
FIG. 12 is a diagram illustrating an eighth configuration of the
optical amplifier.
DESCRIPTION OF EMBODIMENTS
As described previously, in Patent Document 1, additionally, the
pump laser of the optical amplifier is controlled depending on the
delay time of the delay line. The optical amplifier in Patent
Document 1 causes a considerable loss because it requires the delay
line.
Embodiments will now be described with reference to the drawings.
The configurations of the embodiments are merely illustrative; the
present technique is not limited to the configurations of the
embodiments.
FIG. 1 illustrates an optical amplifier 1 applied to an optical
communication system. For example, the optical amplifier 1 is used
for optical wavelength-division multiplexing communication in which
a plurality of optical signals are transmitted at different optical
wavelengths.
The optical amplifier 1 includes a first optical amplifying unit 2,
a second optical amplifying unit 3, a dispersion compensator 12,
and a propagation delay detector 50.
The first optical amplifying unit 2 includes optical couplers 14 to
16, an optical gain medium 11, photodetectors 20 and 22, a laser
21, analog-to-digital conversion circuits (ADC) 26 and 28, a
digital-to-analog conversion circuit (DAC) 27, and an automatic
gain control 32.
The optical coupler 14 bifurcates light from a transmission line
and supplies the bifurcated light to the optical coupler 15 and to
the photodetector 20. The photodetector 20 converts the light from
the optical coupler 14 into an electrical signal and supplies it to
the ADC 26. In other words, the photodetector 20 is a detector for
detecting the optical power input to the optical amplifier 1. The
ADC 26 converts the analog signal from the photodetector 20 into a
digital value and supplies it to the automatic gain control 32.
The optical coupler 15 supplies the light from the optical coupler
14 and pump light from the laser 21 to the optical gain medium
11.
The optical gain medium 11 is a quartz optical fiber doped with a
rare-earth element. For example, the optical gain medium 11 used
may be an erbium-doped quartz optical fiber (EDF). The optical gain
medium 11 amplifies the light coming from the optical coupler 14
with the pump light from the optical coupler 15. The optical gain
medium 11 supplies the amplified light to the optical coupler
16.
The optical coupler 16 bifurcates the light from the optical gain
medium 11 and supplies the bifurcated light to the dispersion
compensator 12 and to the photodetector 22. The photodetector 22
converts the light from the optical coupler 16 into an electrical
signal and supplies it to the ADC 28. The ADC 28 converts the
analog signal from the photodetector 22 into a digital value and
supplies it to the automatic gain control 32.
Based on the values from the ADCs 26 and 28, the automatic gain
control 32 determines the gain of the first optical amplifying unit
2 from the values corresponding to the optical levels input to and
output from the first optical amplifying unit 2 and controls the
gain so that it remains constant.
The dispersion compensator 12 changes the state of dispersion of
the light supplied from the optical coupler 16 of the first optical
amplifying unit 2 and supplies it to the second optical amplifying
unit 3. The dispersion compensator 12 has a dispersion
corresponding to that of the transmission line. Specifically, the
dispersion compensator 12 has a dispersion opposite in sign to that
of the transmission line so as to cancel it out. The dispersion
compensator 12, however, needs not completely compensate for the
dispersion, depending on the configuration of the optical
communication system, but may have a dispersion overly or
insufficiently compensating for the dispersion of the transmission
line. Generally, the dispersion compensator 12 used is a
dispersion-compensating fiber, although a VIPA device (see U.S.
Pat. No. 5,930,045), for example, may also be used. The amount of
dispersion of the dispersion compensator 12 depends on the distance
over which light propagates through the dispersion compensation
medium. Hence, the propagation distance varies with the value of
dispersion compensation of the dispersion compensator 12.
Accordingly, the dispersion compensator 12 has a delay depending on
the transmission line to which the optical amplifier 1 is
connected. The delay of the dispersion compensator 12 is
sufficiently longer than the relaxation time (about 2 .mu.s). The
relaxation time is the period of time in which electrons of the
rare-earth element in the optical gain medium 11 or 13 fall to an
energy level where they cause stimulated emission with pump
energy.
The second optical amplifying unit 3 includes optical couplers 17
to 19, an optical gain medium 13, photodetectors 23 and 25, a laser
24, ADCs 29 and 31, a DAC 30, and an automatic gain control 33.
The optical coupler 17 bifurcates light from the dispersion
compensator 12 and supplies the bifurcated light to the optical
coupler 18 and to the photodetector 23. The photodetector 23
converts the light from the optical coupler 17 into an electrical
signal and supplies it to the ADC 29. The ADC 29 converts the
analog signal from the photodetector 23 into a digital value and
supplies it to the automatic gain control 33.
The optical coupler 18 supplies the light from the optical coupler
17 and pump light from the laser 24 to the optical gain medium
13.
The optical gain medium 13 is a quartz optical fiber doped with a
rare-earth element. For example, the optical gain medium 13 used
may be EDF. The optical gain medium 13 amplifies the light coming
from the optical coupler 17 with the pump light from the optical
coupler 18. The optical gain medium 13 supplies the amplified light
to the optical coupler 19. The optical gain medium 13 performs
forward-pumped optical amplification because the optical signal
from the optical coupler 17 and the pump light from the laser 24
enters the optical gain medium 13 through the coupler 18.
The optical coupler 19 bifurcates the light from the optical gain
medium 13 and outputs one bifurcated portion as the output light of
the optical amplifier 1 and supplies the other portion to the
photodetector 25. The photodetector 25 converts the light from the
optical coupler 19 into an electrical signal and supplies it to the
ADC 31. The ADC 31 converts the analog signal from the
photodetector 25 into a digital value and supplies it to the
automatic gain control 33.
Based on the values from the ADCs 29 and 31, the automatic gain
control 33 determines the gain of the second optical amplifying
unit 3 from the values corresponding to the optical levels input to
and output from the second optical amplifying unit 3 and controls
the gain so that it remains constant.
The propagation delay detector 50 detects the propagation delay
time between the dispersion compensator 12, or the dispersion
compensator 12 and the first optical amplifying unit 2, and the
second optical amplifying unit 3. A timing determined by
subtracting the relaxation time (about 2 .mu.s) from the
propagation delay time detected by the propagation delay detector
50 is supplied to the automatic gain control 33. The propagation
delay time is determined by deliberately causing an optical
variation in advance at the activation of the optical amplifier 1
and measuring the delay time of the point of change, or by
measuring the delay time of light reception at the input of light.
The automatic gain control 33 performs gain control of the second
optical amplifying unit 3 with a time shift from the timing at
which an optical variation occurred, based on the propagation delay
time minus the relaxation time.
The gain controls 32 and 33 of the first and second optical
amplifying units 2 and 3 separately perform automatic gain control
for flattening the gain of the optical amplifier 1. A flattened
gain is a quality necessary for optical signals at individual
wavelengths to achieve substantially the same power in optical
wavelength-division multiplexing communication.
In the automatic gain control, the outputs of the lasers 21 and 24
are changed by comparing gains calculated from power levels
measured using the input and output power monitors of the optical
amplifying units 2 and 3, with a target value set in advance.
The input power monitor of the first optical amplifying unit 2
corresponds to the optical coupler 14, the photodetector 20, and
the ADC 26. The output power monitor of the first optical
amplifying unit 2 corresponds to the optical coupler 16, the
photodetector 22, and the ADC 28. The input power monitor of the
second optical amplifying unit 3 corresponds to the optical coupler
17, the photodetector 23, and the ADC 29. The output power monitor
of the second optical amplifying unit 3 corresponds to the optical
coupler 19, the photodetector 25, and the ADC 31.
In this case, the pumping powers of the lasers 21 and 24 depend on
the total powers input to the optical amplifiers 2 and 3. This is
because doubling the power of light of one wave and doubling the
power of light of two multiple waves require different pumping
powers for the same gain to be achieved.
The automatic gain control is performed in the gain controls 32 and
33 according to the following formulas: Pout/Pin=G
G-G_target=.DELTA.G dPump/dG=f(Pout) Pump=Pump-dPump/dG*.DELTA.G
where G_target is the target gain, Pin is the input value of the
optical amplifying unit 2 or 3 input from the ADC 26 or 29, Pout is
the output value of the optical amplifying unit 2 or 3 input from
the ADC 28 or 31, G is the actual gain of the optical amplifying
unit 2 or 3, .DELTA.G is the difference between the actual gain and
the target gain, Pump is the value of the pumping power supplied to
the DAC 27 or 30, and dPump/dG is the amount of change in pumping
power required for a change in gain (where dPump/dG is represented
as a function f because it depends on the output power Pout).
That is, the automatic gain control 33 performs automatic gain
control by subtracting the ratio of a change in current pumping
power Pump to a change in gain, namely, dPump/dG, multiplied by the
difference from the target gain, namely, .DELTA.G. The arithmetic
of the above automatic gain control is implemented by a
microprocessor.
FIGS. 2A to 2D illustrate time charts of the gain control of the
optical amplifying unit 3 in the embodiment of FIG. 1. FIG. 2A
illustrates the optical power input to the optical amplifier 1.
FIG. 2B illustrates the output of the optical amplifier 1 in the
case where the embodiment is not employed. FIG. 2C illustrates the
pumping power of the laser 24. FIG. 2D illustrates the output of
the optical amplifier 1 in the case where the embodiment is
employed.
At the timing indicated by the arrow in FIG. 2A, the number of
wavelengths of input optical signals (optical channels) changes;
this example illustrates the case where the power drops by 15 dB.
The light changed at the timing indicated by the arrow in FIG. 2A
is delayed in the dispersion compensator 12 by a time .tau. and is
input to the optical amplifying unit 3. In this case, as
illustrated in FIG. 2C, the output of the laser 24 is changed with
a time shift equal to the delay time .tau. minus the relaxation
time. As a result, as illustrated in FIG. 2D, a momentary variation
occurs only at the gap between the pump control timing and the
variation output timing. As for the momentary variation time, the
output may be continued without a level variation if ideal time
adjustment is completely achieved. Even if there is a slight timing
difference, the pumping intensity of the pump laser 24 may be
lowered in advance to provide the effect of alleviating a level
variation.
FIG. 3 illustrates a first configuration of the optical amplifier
1. In FIG. 3, the same members as in the configuration in FIG. 1
are indicated by the same reference numerals, and a description
thereof will be omitted.
The propagation delay detector 50 includes a first edge-detecting
circuit 51, a second edge-detecting circuit 52, a delay-time
counter 53, a relaxation-time subtracter 54, a register 55, and a
delay 56.
The first edge-detecting circuit 51 outputs a signal starting the
delay-time counter 53 when detecting a change in the electrical
signal from the photodetector 20. The second edge-detecting circuit
52 outputs a signal stopping the delay-time counter 53 when
detecting a change in the electrical signal from the photodetector
23.
The delay-time counter 53 starts counting when receiving the signal
from the first edge-detecting circuit 51 and stops counting when
receiving the signal from the second edge-detecting circuit 52. The
delay-time counter 53 supplies the count to the relaxation-time
subtracter 54. The delay-time counter 53 counts the propagation
delay time from the optical coupler 14 to the optical coupler 17.
That is, the delay-time counter 53 counts the propagation delay
time in the stage preceding the second optical amplifying unit 3,
including the dispersion compensator 12.
The relaxation-time subtracter 54 subtracts the relaxation time
from the propagation delay time supplied from the delay-time
counter 53. The relaxation time herein refers to the period of time
in which electrons of the rare-earth element in the optical gain
medium 13 fall to an energy level where they cause stimulated
emission with the pump light of the optical amplifying unit 3. For
example, the relaxation time for pump light with a wavelength of
980 nm is 2 .mu.s to 3 .mu.s, and the relaxation time for pump
light with a wavelength of 1,480 nm is zero. Accordingly, a
relaxation time of 2 .mu.s to 3 .mu.s may be subtracted from the
propagation delay time for pump light with a wavelength of 980 nm,
whereas a relaxation time of zero may be subtracted from the
propagation delay time for pump light with a wavelength of 1,480
nm. The relaxation-time subtracter 54 supplies the arithmetic
result to the register 55.
The register 55 holds the arithmetic result from the
relaxation-time subtracter 54 and fixes the amount of delay of the
delay 56 based on the arithmetic result. The delay 56 delays the
signal supplied from the first edge-detecting circuit 51 by the
time held in the register 55. The delay 56 supplies the delayed
signal to the gain control 40.
The gain control 40 includes a laser controller 41, a
variation-holding circuit 42, and a microprocessor 33'. The
variation-holding circuit 42 detects the amount of change .DELTA.P
in the optical power input to the first optical amplifying unit 2.
FIG. 4 illustrates the configuration of the variation-holding
circuit 42 in FIG. 3. The variation-holding circuit 42 includes a
peak-detecting circuit 421, a bottom-detecting circuit 422, and an
arithmetic circuit 423.
The peak-detecting circuit 421 detects the peak value of the signal
from the photodetector 20, which serves as the input monitor of the
first optical amplifying unit 2. The peak-detecting circuit 421
supplies the detection result to the arithmetic circuit 423. The
bottom-detecting circuit 422 detects the bottom value (floor value)
of the signal from the photodetector 20, which serves as the input
monitor of the first optical amplifying unit 2. The
bottom-detecting circuit 422 supplies the detection result to the
arithmetic circuit 423. The peak-detecting circuit 421 and the
bottom-detecting circuit 422 detect and hold a variation by
comparison with the power level in a normal state. Accordingly, the
power level of the peak-detecting circuit 421 and the
bottom-detecting circuit 422 is set a predetermined period of time
after adjustment of the pump laser 24. The above operation of the
peak-detecting circuit 421 and the bottom-detecting circuit 422 is
intended to alleviate a variation in the output level of the
optical amplifier 1 after a variation in the input level of the
optical amplifier 1 occurs.
The transient response needs very high speed gain control. If the
gain control is too fast, it may create fluctuation in actual
network. As long as the fast response works only when the power
variation is beyond the thresholds, we may avoid the unstable
fluctuation by normal power variation. In FIG. 4, the high speed
transient response works only when the power variation is too
large. It means that the high speed transient control has "the
deadband" which means the insensitive range for the power
variation. If the power variation is small, the residual channel
may not cause any problem.
The signal Set may be asserted by the microprocessor 33' after
activation of the pump laser 24. This may be executed, for example,
at the timing when all Trg signals in FIG. 5 return to a normal
automatic gain control state.
The arithmetic circuit 423 detects the amount of change .DELTA.P in
the optical power input to the first optical amplifying unit 2
based on the values supplied from the peak-detecting circuit 421
and the bottom-detecting circuit 422. The arithmetic circuit 423
used may be an operational amplifier.
FIG. 5 illustrates the configuration of the microprocessor 33' and
the laser controller 41 in FIG. 3. The microprocessor 33' functions
as the automatic gain control 33 in FIG. 1. In addition, the
microprocessor 33' functions as a change-in-pumping-power
calculating section 49 that calculates a change in pumping power,
.DELTA.pump, from the change in signal, .DELTA.P. In addition, the
microprocessor 33' functions as a pumping-power calculating section
48 that calculates the pumping power, pump+.DELTA.pump,
corresponding to the change in the input signal to the optical
amplifier 1 by adding the change in pumping power, .DELTA.pump, to
the pumping power. In addition, the microprocessor 33' functions as
a register 47 that temporarily holds the value of pumping power
supplied to a DAC 30'.
The automatic gain control 33 performs the arithmetic operation
described with reference to FIG. 1. The change-in-pumping-power
calculating section 49 determines the change in pumping power,
.DELTA.pump, from the values used in the operation of the automatic
gain control 33, that is, the input of the optical amplifying unit
3, namely, Pin, and the ratio of a change in current pumping power,
pump, to a change in gain, namely, dPump/dG, and from the output of
the variation-holding circuit 42, namely, .DELTA.P, by the
following arithmetic: .DELTA.pump=.DELTA.P/Pin.times.dPump/dG
The laser controller 41 includes a first high-impedance circuit 44,
a second high-impedance circuit 45, DACs 30' and 30'', an ADC 46,
and an inverter 43.
The DAC 30' converts the value of pumping power, pump, from the
register 47 into an analog value and supplies it to the first
high-impedance circuit 44. The first high-impedance circuit 44
controls the output of the DAC 30' in response to a control signal
from the inverter 43.
The DAC 30'' converts the value of pumping power, pump+.DELTA.pump,
from the pumping-power calculating section 48 into an analog value
and supplies it to the second high-impedance circuit 45. The second
high-impedance circuit 45 controls the output of the DAC 30'' in
response to the trigger signal (Trg or Trigger) from the
propagation delay detector 50.
The inverter 43 inverts the trigger signal (Trg or Trigger) from
the propagation delay detector 50 and supplies it to the first
high-impedance circuit 44.
Thus, if the trigger signal (Trg or Trigger) from the propagation
delay detector 50 is "1", the output of the laser controller 41 is
the output of the second high-impedance circuit 45. If the trigger
signal (Trg or Trigger) from the propagation delay detector 50 is
"0", the output of the laser controller 41 is the output of the
first high-impedance circuit 44. The ADC 46 digitizes the output of
the laser controller 41 and supplies it to the microprocessor 33'.
The delayed trigger signal from the propagation delay detector 50
may be input to the laser controller 41 either directly or via the
microprocessor 33'.
The microprocessor 33', which does not operate so fast that it may
follow a level variation, constantly uses the output of the laser
controller 41 for automatic gain control; that is, the
microprocessor 33' (level-variation controller) constantly reads
the laser control value set for alleviating the level variation of
the optical amplifier 1. After a level variation occurs, the
automatic gain control starts from the set laser control value.
FIG. 6 illustrates a second configuration of the optical amplifier
1. In FIG. 6, the same members as in the configuration in FIG. 3
are indicated by the same reference numerals, and a description
thereof will be omitted. The configuration in FIG. 6 differs in the
configuration of the variation-holding circuit 42. While the
variation-holding circuit 42 in FIG. 3 is formed of an analog
circuit, the variation-holding circuit 42 in the example
illustrated in FIG. 6 is formed of a digital circuit. The
variation-holding circuit 42 includes registers 425 and 426, an
arithmetic circuit 427, and a pulse-generating circuit 424.
The register 425 holds the value from the DAC 26. The register 426
holds the value from the register 425. The pulse-generating circuit
424 outputs a plurality of pulses to the registers 425 and 426 for
a predetermined period of time. In response to the pulses from the
pulse-generating circuit 424, the register 425 supplies the value
that it holds to the register 426 and the arithmetic circuit 427.
The arithmetic circuit 427 calculates the amount of change .DELTA.P
based on the values from the registers 425 and 426. The arithmetic
circuit 427 supplies the output .DELTA.P to the microprocessor 33'.
The microprocessor 33' and the laser controller 41 perform the gain
control of the second optical amplifying unit 3 based on the
delayed trigger signal from the propagation delay detector 50.
FIG. 7 illustrates a third configuration of the optical amplifier
1. In FIG. 7, the same members as in the configuration in FIG. 3
are indicated by the same reference numerals, and a description
thereof will be omitted. The configuration in FIG. 7 differs from
that in FIG. 3 in that the second edge-detecting circuit 52 of the
propagation delay detector 50 detects the signal, from the
photodetector 25, serving as the output of the second optical
amplifying unit 3. That is, the delay-time counter 53 counts the
propagation delay time up to the output stage of the second optical
amplifying unit 3, including the dispersion compensator 12.
In FIG. 7, in addition to the forward-pumping laser 24, the second
optical amplifying unit 3 includes a backward-pumping laser 24'.
The backward-pumping laser 24' supplies pump light to the optical
gain medium 13 via an optical coupler 18'. The optical gain medium
11 is bidirectionally pumped with the pump light from the lasers 24
and 24'.
The laser 24 is subjected only to automatic gain control by a
microprocessor 33''. The laser 24' is controlled by a laser
controller 41' and the microprocessor 33''. The laser controller
41' operates in the same way as the laser controller 41 in FIG. 5.
The microprocessor 33'' operates in the same way as the
microprocessor 33' in FIG. 5. That is, the laser 24' is subjected
to gain control corresponding to variations in input optical
power.
The second optical amplifying unit 3 in the third configuration may
employ the forward-pumping configuration in FIG. 3 except that the
second edge-detecting circuit 52 of the propagation delay detector
50 detects the signal, from the photodetector 25, serving as the
output of the second optical amplifying unit 3.
FIG. 8 illustrates a fourth configuration of the optical amplifier
1. In FIG. 8, the same members as in the configuration in FIG. 7
are indicated by the same reference numerals, and a description
thereof will be omitted. In FIG. 8, the forward-pumping laser 24 of
the optical amplifying unit 3 is also subjected to gain control
corresponding to variations in input optical power by the laser
controller 41 and the microprocessor 33''.
The propagation delay detector 50 includes an edge-detecting
circuit 51, an edge-detecting circuit 52, a delay-time counter 53,
relaxation-time subtracters 54a and 54b, registers 55a and 55b, and
delays 56a and 56b.
The edge-detecting circuit 51 outputs a signal starting the
delay-time counter 53 when detecting a change in the electrical
signal from the photodetector 20. The edge-detecting circuit 52
outputs a signal stopping the delay-time counter 53 when detecting
a change in the electrical signal from the photodetector 23.
The delay-time counter 53 starts counting when receiving the signal
from the edge-detecting circuit 51 and stops counting when
receiving the signal from the edge-detecting circuit 52. The
delay-time counter 53 supplies the count to the relaxation-time
subtracters 54a and 54b. The delay-time counter 53 counts the
propagation delay time from the optical coupler 14 to the optical
coupler 17. That is, the delay-time counter 53 counts the
propagation delay time in the stage preceding the second optical
amplifying unit 3, including the dispersion compensator 12.
The relaxation-time subtracters 54a and 54b subtract the relaxation
time from the propagation delay time supplied from the delay-time
counter 53. The relaxation time herein refers to the period of time
in which electrons of the rare-earth element fall to an energy
level where they cause stimulated emission with the pump light of
the optical amplifying unit 3. For example, the relaxation time for
pump light with a wavelength of 980 nm is 2 .mu.s to 3 .mu.s, and
the relaxation time for pump light with a wavelength of 1,480 nm is
zero. Accordingly, a relaxation time of 2 .mu.s to 3 .mu.s may be
subtracted from the propagation delay time for pump light with a
wavelength of 980 nm. The relaxation-time subtracters 54a and 54b
supply the arithmetic results to the registers 55a and 55b,
respectively.
The registers 55a and 55b hold the arithmetic results from the
relaxation-time subtracters 54a and 54b and fix the amounts of
delay of the delays 56a and 56b, respectively, based on the
arithmetic results. The delays 56a and 56b delay the signal
supplied from the edge-detecting circuit 51 by the time held in the
registers 55a and 55b. The delays 56a and 56b supply the delayed
signals to the gain control 40. The trigger signal from the delay
56a is used to control the laser controller 41. The trigger signal
from the delay 56b is used to control the laser controller 41'.
FIG. 9 illustrates a fifth configuration of the optical amplifier
1. In FIG. 9, the same members as in the configuration in FIG. 8
are indicated by the same reference numerals, and a description
thereof will be omitted. The configuration in FIG. 9 differs from
that in FIG. 8 in the configuration of the propagation delay
detector 50 and correspondingly in the control by the
microprocessor 33''.
The propagation delay detector 50 includes edge-detecting circuits
51, 52a, and 52b, delay-time counters 53a and 53b, relaxation-time
subtracters 54a and 54b, registers 55a and 55b, and delays 56a and
56b.
The edge-detecting circuit 51 outputs a signal starting the
delay-time counter 53a when detecting a change in the electrical
signal from the photodetector 20. The edge-detecting circuit 52a
outputs a signal stopping the delay-time counter 53a when detecting
a change in the electrical signal from the photodetector 23. The
delay-time counter 53a starts counting when receiving the signal
from the edge-detecting circuit 51 and stops counting when
receiving the signal from the edge-detecting circuit 52a. The
delay-time counter 53a supplies the count to the relaxation-time
subtracter 54a.
The relaxation-time subtracter 54a subtracts the relaxation time
from the propagation delay time supplied from the delay-time
counter 53a. The relaxation time herein refers to the period of
time in which electrons of the rare-earth element fall to an energy
level where they cause stimulated emission with the pump light of
the optical amplifying unit 3. For example, the relaxation time for
pump light with a wavelength of 980 nm is 2 .mu.s to 3 .mu.s, and
the relaxation time for pump light with a wavelength of 1,480 nm is
zero. Accordingly, a relaxation time of 2 .mu.s to 3 .mu.s may be
subtracted from the propagation delay time for pump light with a
wavelength of 980 nm. The relaxation-time subtracter 54a supplies
the arithmetic result to the register 55a.
The register 55a holds the arithmetic result from the
relaxation-time subtracter 54a and fixes the amount of delay of the
delay 56a based on the arithmetic result. The delay 56a delays the
signal supplied from the edge-detecting circuit 51 by the time held
in the register 55a. The delay 56a supplies the delayed signal to
the gain control 40. The trigger signal from the delay 56a is used
to control the laser controller 41.
The delay-time counter 53a counts the propagation delay time from
the optical coupler 14 to the optical coupler 17. That is, the
delay-time counter 53a counts the propagation delay time in the
stage preceding the second optical amplifying unit 3, including the
dispersion compensator 12.
The edge-detecting circuit 51 outputs a signal starting the
delay-time counter 53b when detecting a change in the electrical
signal from the photodetector 20. The edge-detecting circuit 52b
outputs a signal stopping the delay-time counter 53b when detecting
a change in the electrical signal from the photodetector 25. The
delay-time counter 53b starts counting when receiving the signal
from the edge-detecting circuit 51 and stops counting when
receiving the signal from the edge-detecting circuit 52b. The
delay-time counter 53b supplies the count to the relaxation-time
subtracter 54b.
The relaxation-time subtracter 54b subtracts the relaxation time
from the propagation delay time supplied from the delay-time
counter 53b. The relaxation time herein refers to the period of
time in which electrons of the rare-earth element fall to an energy
level where they cause stimulated emission with the pump light of
the optical amplifying unit 3. For example, the relaxation time for
pump light with a wavelength of 980 nm is 2 .mu.s to 3 .mu.s, and
the relaxation time for pump light with a wavelength of 1,480 nm is
zero. Accordingly, a relaxation time of 2 .mu.s to 3 .mu.s may be
subtracted from the propagation delay time for pump light with a
wavelength of 980 nm. The relaxation-time subtracter 54b supplies
the arithmetic result to the register 55b.
The register 55b holds the arithmetic result from the
relaxation-time subtracter 54b and fixes the amount of delay of the
delay 56b based on the arithmetic result. The delay 56b delays the
signal supplied from the edge-detecting circuit 51 by the time held
in the register 55b. The delay 56b supplies the delayed signal to
the gain control 40. The trigger signal from the delay 56b is used
to control the laser controller 41.
The delay-time counter 53b counts the propagation delay time from
the optical coupler 14 to the optical coupler 19. That is, the
delay-time counter 53b counts the propagation delay time up to the
output stage of the second optical amplifying unit 3, including the
dispersion compensator 12.
The microprocessor 33'' has the same function as the microprocessor
33' in FIG. 5. However, because the laser controllers 41 and 41'
are provided in FIG. 9, the function in FIG. 5 is provided for each
of the laser controllers 41 and 41'. The laser controller 41 is
supplied with the trigger signal from the delay 56a directly or via
the microprocessor 33''. The laser controller 41' is supplied with
the trigger signal from the delay 56b directly or via the
microprocessor 33''.
FIG. 10 illustrates a sixth configuration of the optical amplifier
1. In FIG. 10, the same members as in the configuration in FIG. 7
are indicated by the same reference numerals, and a description
thereof will be omitted. The configuration in FIG. 10 differs from
the other configurations in that the gain adjustment is performed
by an optical attenuator 49.
The optical attenuator 49 is disposed between the optical gain
medium 13 and the optical coupler 19. The laser 24 is subjected
only to automatic gain control by the microprocessor 33''. The
second edge-detecting circuit 52 of the propagation delay detector
50 detects the signal, from the photodetector 25, serving as the
output of the second optical amplifying unit 3.
The microprocessor 33'' operates in the same way as the
microprocessor 33' in FIG. 5. However, the amount of attenuation
.DELTA.pump is supplied to the optical attenuator 49.
FIG. 11 illustrates a seventh configuration of the optical
amplifier 1. In FIG. 11, the same members as in the configuration
in FIG. 3 are indicated by the same reference numerals, and a
description thereof will be omitted. The configuration in FIG. 11
differs from that in FIG. 3 in that a pump-light control timing
circuit 60 is added. The pump-light control timing circuit 60 is a
circuit for synchronizing the timings of automatic gain control
performed in the first and second optical amplifying units 2 and 3
by taking into account the delay time of the dispersion compensator
12.
The pump-light control timing circuit 60 includes a pump-light
control timing pulse circuit 61 and a delay circuit 62.
The pump-light control timing pulse circuit 61 is a circuit for
generating a control pulse for providing the timing for pumping
operation of the first and second optical amplifying units 2 and 3.
The delay circuit 62 delays the pulse output from the pump-light
control timing pulse circuit 61 according to the value in the
register 55.
The microprocessor 33' controls the laser controller 41 based on
the automatic gain control timing from the delay circuit 62.
FIG. 12 illustrates an eighth configuration of the optical
amplifier 1. In FIG. 12, the same members as in the configuration
in FIG. 10 are indicated by the same reference numerals, and a
description thereof will be omitted. The configuration in FIG. 12
differs from that in FIG. 10 in that a pump-light control timing
circuit 60 is added. The pump-light control timing circuit 60
operates in the same way as that in FIG. 11.
Various features partially selected from the first to eighth
configurations of the optical amplifier 1 may be removed or
combined as needed.
In the optical amplifier 1, optical amplification is not essential
for the first optical amplifying unit 2; it only needs to monitor
the light input to the optical amplifier 1. In the first to eighth
configurations of the optical amplifier 1 of the optical amplifier
1, therefore, the first optical amplifying unit 2 may have a
configuration excluding the components other than the optical
coupler 14 and the photodetector 20.
All examples and conditional language recited herein are intended
for pedagogical purposes to aid the reader in understanding the
embodiment and the concepts contributed by the inventor to
furthering the art, and are to be construed as being without
limitation to such specifically recited examples and conditions,
nor does the organization of such examples in the specification
relate to a illustrating of the superiority and inferiority of the
embodiment. Although the embodiments have been described in detail,
it should be understood that the various changes, substitutions,
and alterations could be made hereto without departing from the
spirit and scope of the invention.
* * * * *